A method and apparatus for producing an extruded polystyrene foam is disclosed in German Patent Specification No. DE 33 10 751A1. The apparatus disclosed in this prior document comprises an extruder, the interior of which is sub-divided into a plurality of zones. Initially, a mixture of polystyrene and a nucleating agent is fed into such an apparatus and is fused in a fusion zone. The fused mass then enters a zone in which the pressure of the fused mass is caused to increase. Thereafter, the fused mass passes into an injection zone wherein it has a propellant and water injected thereinto. Finally, the mixture passes into a cooling and discharge zone from which it is subsequently extruded through an annular nozzle. There is thus formed a hose, which is subsequently cut open, laid flat and wound. Such method produces a polystyrene foam, the cell structure of which is a combination of large and small cells.
A structure having a uniform cell size cannot be produced utilising this known method. This is because it is not possible to ensure a uniform distribution of the injected water in the hot fused mass of plastics material, which latter generally has a temperature lying in the range of from 150.degree. to 250.degree. C.